[PATCH v11 44/56] media: core: Report the maximum possible number of buffers for the queue

[Hans Verkuil]

On 12/10/2023 13:46, Benjamin Gaignard wrote:
> Use one of the struct v4l2_create_buffers reserved bytes to report
> the maximum possible number of buffers for the queue.
> V4l2 framework set V4L2_BUF_CAP_SUPPORTS_SET_MAX_BUFS flags in queue

Ah, that's not a correct cap name. That suggests that userspace can
set the max number of buffers. Instead it should be:

V4L2_BUF_CAP_SUPPORTS_MAX_NUM_BUFFERS

indicating that userspace can use the max_num_buffers field.

> capabilities so userland can know when the field is valid.
> 
> Signed-off-by: Benjamin Gaignard <benjamin.gaignard at collabora.com>
> 
> # Conflicts:
> #	drivers/media/common/videobuf2/videobuf2-v4l2.c
> ---
>  .../userspace-api/media/v4l/vidioc-create-bufs.rst        | 8 ++++++--
>  Documentation/userspace-api/media/v4l/vidioc-reqbufs.rst  | 1 +
>  drivers/media/common/videobuf2/videobuf2-v4l2.c           | 2 ++
>  drivers/media/v4l2-core/v4l2-ioctl.c                      | 4 ++--
>  include/uapi/linux/videodev2.h                            | 7 ++++++-
>  5 files changed, 17 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/userspace-api/media/v4l/vidioc-create-bufs.rst b/Documentation/userspace-api/media/v4l/vidioc-create-bufs.rst
> index a048a9f6b7b6..1a46549e7462 100644
> --- a/Documentation/userspace-api/media/v4l/vidioc-create-bufs.rst
> +++ b/Documentation/userspace-api/media/v4l/vidioc-create-bufs.rst
> @@ -116,9 +116,13 @@ than the number requested.
>        - ``flags``
>        - Specifies additional buffer management attributes.
>  	See :ref:`memory-flags`.
> -
>      * - __u32
> -      - ``reserved``\ [6]
> +      - ``max_buffers``

Call it max_num_buffers. It clearly indicates that it is about the maximum
number of buffers, and not e.g. maximum buffer size or some other buffer
property. It is also consistent with the internal field name.

> +      - If V4L2_BUF_CAP_SUPPORTS_SET_MAX_BUFS capability flag is set
> +        this field indicate the maximum possible number of buffers
> +        for this queue.
> +    * - __u32
> +      - ``reserved``\ [5]
>        - A place holder for future extensions. Drivers and applications
>  	must set the array to zero.
>  
> diff --git a/Documentation/userspace-api/media/v4l/vidioc-reqbufs.rst b/Documentation/userspace-api/media/v4l/vidioc-reqbufs.rst
> index 099fa6695167..0395187e1a5a 100644
> --- a/Documentation/userspace-api/media/v4l/vidioc-reqbufs.rst
> +++ b/Documentation/userspace-api/media/v4l/vidioc-reqbufs.rst
> @@ -120,6 +120,7 @@ aborting or finishing any DMA in progress, an implicit
>  .. _V4L2-BUF-CAP-SUPPORTS-ORPHANED-BUFS:
>  .. _V4L2-BUF-CAP-SUPPORTS-M2M-HOLD-CAPTURE-BUF:
>  .. _V4L2-BUF-CAP-SUPPORTS-MMAP-CACHE-HINTS:
> +.. _V4L2-BUF-CAP-SUPPORTS-SET-MAX-BUFS:
>  
>  .. raw:: latex
>  
> diff --git a/drivers/media/common/videobuf2/videobuf2-v4l2.c b/drivers/media/common/videobuf2/videobuf2-v4l2.c
> index 799dd43b4aa9..5f4ea4485223 100644
> --- a/drivers/media/common/videobuf2/videobuf2-v4l2.c
> +++ b/drivers/media/common/videobuf2/videobuf2-v4l2.c
> @@ -681,6 +681,7 @@ EXPORT_SYMBOL(vb2_querybuf);
>  static void fill_buf_caps(struct vb2_queue *q, u32 *caps)
>  {
>  	*caps = V4L2_BUF_CAP_SUPPORTS_ORPHANED_BUFS;
> +	*caps |= V4L2_BUF_CAP_SUPPORTS_SET_MAX_BUFS;
>  	if (q->io_modes & VB2_MMAP)
>  		*caps |= V4L2_BUF_CAP_SUPPORTS_MMAP;
>  	if (q->io_modes & VB2_USERPTR)
> @@ -763,6 +764,7 @@ int vb2_create_bufs(struct vb2_queue *q, struct v4l2_create_buffers *create)
>  	fill_buf_caps(q, &create->capabilities);
>  	validate_memory_flags(q, create->memory, &create->flags);
>  	create->index = vb2_get_num_buffers(q);
> +	create->max_buffers = q->max_num_buffers;
>  	if (create->count == 0)
>  		return ret != -EBUSY ? ret : 0;
>  
> diff --git a/drivers/media/v4l2-core/v4l2-ioctl.c b/drivers/media/v4l2-core/v4l2-ioctl.c
> index 9b1de54ce379..da355355a869 100644
> --- a/drivers/media/v4l2-core/v4l2-ioctl.c
> +++ b/drivers/media/v4l2-core/v4l2-ioctl.c
> @@ -483,9 +483,9 @@ static void v4l_print_create_buffers(const void *arg, bool write_only)
>  {
>  	const struct v4l2_create_buffers *p = arg;
>  
> -	pr_cont("index=%d, count=%d, memory=%s, capabilities=0x%08x, ",
> +	pr_cont("index=%d, count=%d, memory=%s, capabilities=0x%08x, max buffers=%u",
>  		p->index, p->count, prt_names(p->memory, v4l2_memory_names),
> -		p->capabilities);
> +		p->capabilities, p->max_buffers);
>  	v4l_print_format(&p->format, write_only);
>  }
>  
> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> index c3d4e490ce7c..ef1402537395 100644
> --- a/include/uapi/linux/videodev2.h
> +++ b/include/uapi/linux/videodev2.h
> @@ -1035,6 +1035,7 @@ struct v4l2_requestbuffers {
>  #define V4L2_BUF_CAP_SUPPORTS_ORPHANED_BUFS		(1 << 4)
>  #define V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF	(1 << 5)
>  #define V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS		(1 << 6)
> +#define V4L2_BUF_CAP_SUPPORTS_SET_MAX_BUFS		(1 << 7)
>  
>  /**
>   * struct v4l2_plane - plane info for multi-planar buffers
> @@ -2605,6 +2606,9 @@ struct v4l2_dbg_chip_info {
>   * @flags:	additional buffer management attributes (ignored unless the
>   *		queue has V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS capability
>   *		and configured for MMAP streaming I/O).
> + * @max_buffers: if V4L2_BUF_CAP_SUPPORTS_SET_MAX_BUFS capability flag is set
> + *		 this field indicate the maximum possible number of buffers
> + *		 for this queue.
>   * @reserved:	future extensions
>   */
>  struct v4l2_create_buffers {
> @@ -2614,7 +2618,8 @@ struct v4l2_create_buffers {
>  	struct v4l2_format	format;
>  	__u32			capabilities;
>  	__u32			flags;
> -	__u32			reserved[6];
> +	__u32			max_buffers;
> +	__u32			reserved[5];
>  };
>  
>  /*

Regards,

	Hans